The use of a continuous roving or tow in the formation of such articles as filters is well known. A tow comprises a plurality of individual strands, often formed of a non-woven, fibrous material, which are packed together in an elongated bundle. The individual strands may extend continuously along the entire length of the tow, or may be a few inches in length in which case they are arranged end-to-end within the tow.
In the fabrication of filters, for example, it is desirable to coat the individual strands forming the tow with a material which improves the filtration properties of the strands, and/or with a material which adheres the individual strands together within the tow. In prior art systems such as disclosed, for example, in U.S. Pat. Nos. 4,317,425 to Greve et al and No. 4,421,055 to Arthur et al, coating material has been applied by axially moving the tow past one or more fixed spraying devices positioned within a chamber having a recovery system for collecting oversprayed coating material. These spraying devices comprise rotating brushes whose bristles pick up the coating material from a reservoir or supply roller, and then project the coating material in fine droplets onto the moving tow.
The problem with this prior art method has been that the coating material is often not applied to each of the individual strands within the tow. Depending upon the density or tightness with which the individual strands in the tow are packed, the coating material applied in the manner described above may not penetrate into the interior of the tow and cover the outer surface of each of the strands. As a result, only the exterior surface of the tow, or some of the individual strands immediately beneath the outer surface of the tow, are covered with the coating material. The filtration properties of such tows are therefore limited.
In order to more completely coat each of the individual strands within a tow, efforts have been made in the prior art to separate the individual strands in the tow before the coating material is applied. One apparatus of this general type is disclosed, for example, in U.S. Pat. No. 2,966,198 to Wylde. In the Wylde patent, the tow is advanced through a chamber in which a pressurized stream of air creates a turbulence to separate the individual strands of the tow. A liquid coating material is thereafter applied to the individual strands from a tube located within the interior of the chamber which has a discharge orifice positioned downstream from the point at which the pressurized air is applied to the tow.
A problem with the apparatus disclosed in Wylde, and similar apparatus, is that the tube which ejects the coating material is located within the interior of the coating chamber and the tow must pass over and around the tube as it moves through the coating chamber. This could result in damage to the tow, particularly where the individual strands thereof are relatively densely packed.